YSlow is a tool developed by Yahoo! and it examines all the components of your page's performance and offers suggestions for improvements. YSlow is integrated into the Firebug web development tool for Firefox, and for Chrome you can download their extension.
The Yahoo! team has identified 34 rules that affect a web page's performance and YSlow is based on 23 of these 34 rules. Studies have shown that if a developer can improve the following 23 rules, they can enhance web performance from up to 25 to 50 percent.
Minimize HTTP requests
Use a CDN
Avoid an empty
src
orhref
Add expires or a Cache-Control Header
Gzip components
Put style sheets at the top
Put scripts at the bottom
Avoid CSS expressions
Make JavaScript and CSS external
Reduce DNS lookups
Minify JavaScript and CSS
Avoid redirects
Remove duplicate scripts
Configure ETags
Make AJAX cacheable
Use
GET
for AJAX requestsReduce the number of DOM elements
Ensure no 404s
Reduce cookie size
Use cookie-free domains for components
Avoid filters
Do not scale...